Two-color pump-probe experiments in helium using high-order harmonics
Two-color pump-probe experiments in helium using high-order harmonics
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DOI：
10.1140/epjd/e2002-00213-1
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发表时间：
2003-01-01
影响因子：
1.8
通讯作者：
L'Huillier, A
中科院分区：
文献类型：
--
作者：
Johansson, A;Raarup, MK;L'Huillier, A
应用泵浦探测技术测量了激发态的寿命和绝对光电离截面。氦原子的1s2p P-1态和13s2p P-1态分别由在Kr气体射流中产生的可调谐70ps染料激光的第13次谐波和第14次谐波激发。通过吸收能量在1.6和4.5 eV之间的探针光子，这些状态在不同的时间延迟后被电离。tau(1s2p) = 0.57 ns和tau(1s2p) = 1.76 ns的寿命测定精度约为15%。当压力大于6 × 10(-5) mbar时，零延时寿命曲线中离子数量的显著增加归因于He的直接双光子电离、激发态的AC Stark展宽和介质中XUV光的吸收。通过测量电离过程的饱和度，以接近25%的精度确定了阈值区He 1s2p P-1和He 13s2p P-1态的绝对光电离截面。此外，泵浦和探针光束的偏振矢量之间的相对取向的变化使得部分光电离截面的确定成为可能。
A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p P-1 and 1s3p P-1 states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of tau(1s2p) = 0.57 ns and tau(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6 x 10(-5) mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p P-1 and He 1s3p P-1 states in the threshold region are determined by measuring the saturation of the ionization process with a precision of similar to 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections.